Proximity detection system using field effect transistors



Nov. 24, 1970 c. F. KLEIN 3,543,056

PROXIMITY DETECTION SYSTEM USING FIELD EFFECT TRANSISTORS Filed Aug. 7, 1967 D. 0. POWER SUPPLY INVENTOR CAR I; F KLEIN -United States Patent Int. Cl. H03k US. Cl. 307--308 Claims ABSTRACT OF THE DISCLOSURE A sensing wire is disposed in an area to be protected such that movement of an intruder into the area results in an induced charge, either positive or negative, on the Wire. The sensing wire is connected to a pair of alarm circuits and particularly to the gates of a pair of field effect transistors each of which is connected to trigger a silicon controlled rectifier in the corresponding alarm circuit. The field effect transistors are interconnected to a low voltage supply with the gates connected in common to the junction of a pair of bias resistors connected across the supply. The one field effect transistor is an N channel type and includes a bias resistor connected in series with its source terminal to the supply and the other transistor is a P channel type and includes a bias resistor connected in series with its drain terminal to the supply. The silicon controlled rectifiers have their gate circuits connected directly across the corresponding bias resistors. The bias resistor provides a low external impedance path and essentially prevents erroneous turn-on of the controlled rectifier as a result of spurious signals such as internally generated leakage currents and the like.

This invention relates to a detection system and particularly to an improved detection system for detecting selected disturbances in a given area.

Detection devices for protecting given areas against certain types of intrusion may be generally divided into two main classes: (1) a wave energy class wherein energy is transmitted at a given frequency and compared with the frequency and/or amplitude of the reflected energy and (2) a field class wherein an electric field in a given area is disturbed by any changes within the area. In the latter, an electric field is established within the area to be effected with the variation in the electric field resulting from an intruder or foreign objects within the field providing an input to a suitable detector.

The present invention is particularly directed to a disturbed field detection system employing a high impedance sensing means and particularly to an electrostatic sensing system for detecting the approach of an intruder into an area employing a novel solid state circuit coupling the sensing means to an alarm circuit or the like. The coupling circuit may also be applied to combustion detection and the like wherein ionization chambers measure the characteristic of the air and airborne matter and provide a related voltage signal. For example, applicants copending application entitled Ionization Chamber Detection Apparatus which was filed on Dec. 22, 1966, with Ser. No. 604,058, discloses an improved ionization chamber system for detection combustion products with the output of the ionization chamber connected to a solid state control circuit employing a field effect transistor. The advantages of this system are that it permits use of a low voltage power supply for controlling any suitable alarm circuit connected to any power supply including the 3,543,056 Patented Nov. 24, 1970 conventional power supply line system employed in this country.

In accordance with the present invention, an intrusion detection system intended to protect against movement of a person into the area includes a sensing antenna in the area. Any movement of an intruder into the area results in the inducement of a charge on the antenna due to the static charge on a body. The induced charge on the sensing wire, which may be either negative or positive, is converted to a related potential or voltage which is then ap plied to a suitable response circuit and preferably the gate of the field effect transistor which triggers a silicon controlled rectifier or the like in an alarm circuit as hereinafter described.

In accordance with the novel coupling circuit aspect of the present invention, each field effect transistor is interconnected to a low voltage supply and its gate element interconnected to the high impedance sensing device. A triggered switch such as a silicon controlled rectifier has a trigger or gate circuit conductively connected directly across the bias resistor. The bias resistor provides a low external impedance path and essentially prevents erroneous turn-on of the controlled rectifier as a result of spurious signals such as internally generated leakage currents and the like.

In accordance with a particularly novel aspect of the present invention, the antenna is connected to a pair of opposite type or polarity sensitive field effect transistors for actuation of different alarm or signal means. The field effect transistor which is responsive to a positive signal is connected to the power supply in a source follower circuit having a bias resistor in series with the source terminal. The field effect transistor which is responsive to a negative signal is connected to the power supply in a source stabilized amplifier circuit with the bias resistor in series with the drain terminal. The triggered alarms are connected across the corresponding bias resistors.

The antenna may be a single or multiple wire system depending upon the size of the area to be protected. Further, where metallic objects such as a safe vault, a metal cabinet or the like is to be protected, the object itself may form the antenna.

The present invention provides an improved means to sense entrance of a person into an invalid area and a reliable solid state detection circuit for triggering of an alarm in response to the output of the high impedance sensing such as results from the movement of an intruder into an area adjacent an object to be protected and the like.

The drawing furnished herewith illustrates preferred constructions of the present invention in which the above advantages and features are clearly disclosed as well as others which will be clear from the following description of the drawing.

The drawing is a schematic circuit diagram of the present invention applied to protection of a given device against unwarranted intrusion and employing the direct coupling of silicon controlled rectifier alarm circuits to a pair of field effect transistors.

Referring to the drawing, the intrusion detection system of the present invention shown in the drawing generally includes a pair of detection or antenna wires 1 and 2 defining an antenna suitably located in an area to be protected against unwarranted intrusion and connected to a direct current source such that an electric field is created between the detection wires 1 and 2 and a suitable reference such as ground. The detection wires 1 and 2 are connected as a signalling input to a pair of sensing or coupling circuits 3 and 4 which in turn are directly connected to trigger related alarm circuits 5 and 6. The alarm circuits may employ a visual, audible or any other desired response means and no further description thereof is given.

The illustrated coupling circuits 3' and 4 are basically similar circuits each of which includes one of a pair of appropriate field effect transistors 7 and 8 such that circuit 3 responds to a selected positive static charge on the antenna and circuit 4 responds if a selected negative charge is placed on the antenna. For purposes of simplicity and clarity of explanation, circuit 3 will be described in detail with the corresponding elements of circuit 4 identified by similar primed numbers and further described as required for a full understanding of the illustrated embodiment of the invention.

Referring particularly to circuit 3, a field effect transistor 7 has its main interbase elements or electrodes 9 and 10 connected to a suitable direct current or supply 11 including a positive D.C. line 12 and a common ground line 13. A source resistor 14 is connected in series with a portion of a potentiometer 15 between the source electrode 10 and the ground line 13. The potentiometer 15 includes a tap 16 connected directly to the adjacent end of the source resistor 14 to permit adjustment of the proportion of the potentiometer 15 connected in series with the resistor 14. The control or gate electrode 17 of the field effect transistor 9 is connected to the detection wires 1 and 2 in common with a current limiting resistor 18. A pair of bias resistors 19 and 20 is connected in series between the positive line 12 and the common ground 13 with the common junction 21 forming a common connection to the detection wires 1 and 2 and to the resistor 18 to provide a proper bias on the electrode 17 of transistor 7.

The illustrated field effect transistor 7 is diagrammatically illustrated as a metal oxide semiconductor variety wherein the gate electrode 17 is separated from a semiconductor path existing between the main electrodes. The potential of the gate electrode 17 controls the introduction of the carriers in the appropriate portion of the semiconductor and thereby the conductivity of such path. In the illustrated embodiment, the transistor 7 includes an acceptor or P-type semiconductor material in the control path and thus the conductivity of the field effect trasistor 7 will increase in response to an increasing positive potential at the gate electrode 12.

The movement of a person into the area of either detection wire 1 or 2 or both, depending upon the location of wires 1 and 2 induces a positive charge on such wire as a result of the static charge on the body of the individual. The positive charge on the antenna provides a signal voltage on the gate electrodes 17 of the field effect transistor 7 causing it to conduct. When the field effect transistor 7 conducts, current flows through the main interbase circuit and establishes a voltage drop across the source resistor 14 which can be applied directly to control the alarm circuit as shown in the drawing.

The alarm circuit 5 includes a silicon controlled rectifier 22 having its anode and cathode elements or terminals connected in series with a load resistor 23 between the positive power line 12 and the tap 16. The gate 24 of controlled rectifier 22 is connected to the top side of the source resistor 14; ie to the junction between the source resistor 14 and the corresponding terminal 10 of the field efiect transistor 7. The alarm circuit is connected across the load resistor 23 and actuated in response to the voltage drop across the resistor.

In operation when the field effect transistor 7 conducts, the voltage drop across the source resistance 14 is direct coupled and establishes a turn-on voltage drop across the gate to cathode circuit of the controlled rectifier 22 sufiicient to trigger the rectifier to conduct. The rectifier will then continue to conduct until the voltage across the anode to cathode circuit is removed or until the holding current drops below a preselected minimum level. The alarm system can be turned off through the use of a suitable switch between the positive line 12 and the input power means or the like.

During the period of standby conditions, the source resistor 14 establishes a low external impedance path across the gate to cathode circuit. This is highly desirable as it prevents transient or spurious triggering of the silicon controlled rectifier 22 or other similar solid state device. Thus, any internally generated leakage currents or other similar false triggering signals are shunted around the gate to cathodev circuit.

The sensing and coupling circuit 4 employes a similar metal oxide semiconductor field effect transistor 8 having an opposite or donor type simiconductor connected in the circuit of the main interbase electrodes 9' and 10 such that a negative potential at gate 17' controls the turn-on of transistor 8. Circuit 4 thus senses a static charge of opposite polarity.

As the transistor 8 is of the opposite polarity, the source electrode 10' is connected directly to tap 16' of potentiometer 15- and the drain electrode 9' is connected in series with a load and drain resistor 25 to the negative side of the power supply 11.

The gate electrode 17' of transistor 8 is connected to the gate electrode 17 of transistor 7 and therefore subject to the same potential. An increasing signal in the positive direction, which as previously noted triggers transistor 7, only serves to further turn off transistor 8. However, an opposite signal, which is an increasing signal in the negative direction, establishes a corresponding potential on the acceptor type field effect transistor 8 which results in introduction of carriers into the semiconductor portion to increase the conductivity and provide an output signal through the drain resistor 25. Therefore, if an intruder or the like enters the area of wire 1 or 2 or engages either of such wires and is so situated as to establish a negative charge on the wire or wires, a corresponding potential is applied to gates 17 and 17'. This results in the triggering of transistor 8 into conduction.

In circuit 4, the gate to cathode circuit of silicon controlled rectifier 22 is connected directly across the drain resistor 25 and functions in the same manner as previously described with respect to the drawing.

In the drawing, a lead 26 is also schematically shown between the lead to source electrode 10 and the casing. This particular field effect transistor is available from General Instrument Co. with an internal protective Zener diode element to protect against damaging of the separating layer for the gate electrode during handling of the transistor and the additional lead is provided for completing the circuit connections. This type of Zener protection in field effect transistors of the donor semiconducting type is not presently commercially available and for this reason transistor 7 is shown without this lead connection.

As previously noted, an electric field is created by the antenna. However, the present invention does not operate on a simple capacitive principle but rather as a result of an induced electrostatic charge resulting from the movement of an intruder in the immediate area of the antenna or into engagement with the antenna. The static charge on the body of the intruder will induce a static charge on the antenna wire. The charge on the antenna wire appears as a change in potential at the gate of the field etfect transistor or other similar amplifying means which triggers the alarm.

In the embodiment of the invention, a pair of wires 1 and 2 is shown for purposes of dsecription. In fact, if a single item is to be protected; for example, a safe or the like, the safe itself can form the detection wire. A pair of wires is shown for purposes of illustrating the multiple wire control system which could be used to cover a door, window, skylight or other enlarged area. If desired, a great number of wires could be used to protect an entire room, building or other area.

As previously noted, the circuit shown might also be applied to an ionization or combustion detection system employing ionization chambers such as shown in the previously referred to copending application of applicant wherein series-connected ionization chambers form a voltage dividing network, the junction of which is connected to the gate of a field effect transistor. The combustion chambers are equivalent to the voltage dividing resistors shown in the above embodiments of the invention and the circuit will function similar to that previously described, with the change in the combustion gases producing the voltage change necessary to trigger the field efiect transistor which in turn directly triggers the silicon controlled rectifier or other similar triggered device.

The illustrated circuit provides detection of both a positive change and a negative change in the electrostatic potential of the sensor. The sensitivity of the circuits 3 and 4 is separately adjustable by the potentiometers 15 and 15'.

Further, if desired, the detection system might be constructed to respond to only a negative or a positive signal by employing either circuit 3 or 4.

The present invention broadly covers the concept of sensing devices having a high direct current resistance such as impedance grids, ionization chambers and high re: sistance humidity sensing elements and the like. In either case the sensors are connected into a voltage division network or bridge circuit to provide a voltage output responsive to changes in a sensed condition for controlling a field elfect transistor. More particularly, the present invention includes a unique combination of an intrusion detection sensing means for triggering of a field efiect transistor or the like.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. Intrusion detection apparatus employing a high impedance sensing means for detecting intrusion into an area, comprising a low voltage direct current power source connection means, an antenna means connected to said direct current power source connection means and defining an electrostatic charge source producing a distinct change in charge responsive to movement of a spaced intruder into the area, and an electrostatic charge detection circuit connected to the power connection means and including an electrostatic charge responsive direct current non-oscillating amplifying means having a field effect transistor with a high input impedance and gate terminal means connected in a bias network to said direct current power source connection means and to said antenna means and having an output means establishing an amplified varying amplitude output corresponding to the electrostatic charge on said antenna means, said high input impedance maintaining the charge of said antenna means and thereby a corresponding output over an appreciable period to provide reliable and accurate detection.

2. The apparatus of claim 1 wherein said field effect transistor is connected to the power source connection means in series with a small resistor and including a triggered switch means having an input circuit means connected across said small resistor whereby said triggered switch means is actuated in accordance with the signal at said antenna wire, said small resistor bypassing spurious signals from said input circuit means.

3. The intrusion detection apparatus of claim 1 wherein said antenna means includes a plurality of spaced wirelike members having one common connected end connected to said input terminal means and a plurality of spaced free ends.

4. The detection apparatus of claim 1 wherein said bias network includes a pair of voltage dividing resistors connected to the power connection means with the common junction connected to said gate terminal means.

5. The detection apparatus of claim 1 wherein said amplifying means includes a second field effect transistor, each of said transistors having a gate as the input terminal means, one of said field effect transistors being constructed to respond to a positive gate signal and the other to respond to a negative gate signal.

6. The detection apparatus of claim 1 wherein said detection circuit includes a pair of sensing circuits, one of said sensing circuits including said field efiect transistor connected as a source stabilized amplifier and having a gate connected to the antenna means to respond to a relatively increasing negative potential at the antenna means, the other of said sensing circuits including a second field eifect transistor connected as a source follower amplifier and having a gate connected to the antenna means to respond to a relatively increasing positive potential at the antenna means.

7. The detection apparatus of claim 1 wherein said detection circuit includes a common power supply, said biasing network including a pair of voltage dividing resistors connected to said power supply and having a common junction, said antenna means being connected to said common junction, a pair of sensing circuits, one of said sensing circuits including said field eifect transistor connected as a source stabilized amplifier to said power supply and including a drain resistor connected in series with a drain electrode and having a gate responding to a relatively increasing negative potential, the other of said sensing circuits including a second field effect transistor connected as a source follower amplifier to said power supply and including a source resistor connected in series with a source electrode and having a gate responsive to a relatively increasing positive potential, a resistor connected between said common junction and both of said gates, and a pair of solid state triggered switch means each having trigger input electrodes, the input electrode of one of said triggered switch means being connected across the drain resistor and the input electrodes of the other being connected across said source resistor.

8. The detection apparatus of claim 1 wherein said detection circuit includes a common power supply, said biasing network including a pair of voltage dividing resistors connected to said power supply and having a common junction, a pair of sensing circuits, one of said sensing circuits including said field eifect transistor connected as a source stabilized amplifier to said power supply and having a gate respnosive to a relatively increasing negative potential, the other of said sensing circuits including a second field effect transistor connected as a source follower amplifier to said power supply and having a gate responsive to a relatively increasing positive potential, and a resistor connected between said common junction and said gates.

9. Detection apparatus, a high impedance sensing means for detecting selected changes in an adjacent area,

a low voltage power source connection means,

a detection circuit including a field efiect transistor having a high input impdeance and input terminal means connected to said high impedance sensing means to control conduction in accordance with the electrostatic charge on said antenna means, said transistor connected to the power source connection means in series with a small resistor, and

a triggered switch means having an input circuit means connected across said small resistor whereby said triggered switch means conducts in response to conduction by said field efiect transistor, said small resistor bypassing spurious signals from said input circuit means.

10. The intrusion detection apparatus of claim 9 wherein said triggered switch means is a controlled rectifier having a gate circuit connected across said small resistor.

References Cited UNITED STATES PATENTS Shephard 34O-258 X Reedyk et al. 307--304 Myers.

Gottlieb 340-258 X Ichimori 340258 Webster 324-149 X 8 v FOREIGN PATENTS 1,009,271 3/1964 Great-Britain.

OTHER REFERENCES Amelco Semiconductor Field Effect Transistors pp.

land 5, No. 1, June, 1962.

JOHN S. HEYMAN, Primary Examiner US. Cl. X.R. 

